The invention relates to a low-noise, broadband amplifier device having at least a first broadband amplifier element having negative feedback. The invention additionally relates to the use of the amplifier device.
Such an amplifier device is employed for example as an amplifier of an ultrasonic apparatus, magnetic resonance apparatus or of a radio-frequency measuring apparatus. In the amplifier device, an electrical signal which, for example, is generated by an ultrasonic transducer in response to a received acoustic signal, may have a very low signal level, and is in this case amplified for subsequent signal processing (not specifically described here). To ensure that the information content is not diminished unnecessarily this amplification should be effected with as little noise as possible.
The reference book U. Tietze, Ch. Schenk, xe2x80x9cHalbleiter-Schaltungstechnikxe2x80x9d [Semiconductor circuitry], Springer-Verlag, 9th edition, 1991, pages 44 to 49 and also 132 to 137 discloses various amplifier devices which include, in addition to at least one active amplifier element, such as a transistor or an operational amplifier, at least one further element for connecting up the amplifier element. In order to eliminate the nonlinearity of the active element and, in particular, also to set a defined gain and input impedance, use is often made here of the circuit principle of negative feedback.
The negative feedback is usually effected via a non-reactive resistor. However, since a non-reactive resistor generates thermal noise, the originally good noise properties of the active amplifier element are significantly impaired by the resistive negative feedback.
In order to avoid this negative effect of resistive negative feedback, inductive transformer-based negative feedback is provided instead in the case of the amplifier devices respectively described in the Company catalog of Adams-Russell Co. Inc. xe2x80x9cRFandMicrowave Signal Processing Componentsxe2x80x9d in the section xe2x80x9cAmplifier Application Notexe2x80x9d on pages 20 and 21 and in U.S. Pat. No. 3,624,536. A very good noise behavior can thus also be achieved for the amplifier device overall, since a transformer is a very low-loss and low-noise element. However, a transformer is relatively expensive and also rather large, so that it can only be integrated with difficulty.
The technical paper xe2x80x9cRauscharme Verstxc3xa4rkerschaltungxe2x80x9d [Low-noise amplifier circuit], Neues aus der Technik, 1979, No. 3, 15.06.1979, p. 2 describes a low-noise amplifier device in which the output of an amplifier element is fed back via a first and a second negative feedback path to the two inputs. One negative feedback path includes a resistor and the other a voltage-controlled voltage source. An inverting differential amplifier is thus produced overall which has an improved noise behavior compared with an inverting differential amplifier connected up in a conventional manner. However, the resistor in the first negative feedback path still supplies a finite contribution to the total noise of the amplifier device. This is because the input of the amplifier device is connected up directly to the resistor.
Moreover, DE 40 24 166 C1 discloses an amplifier device having capacitive negative feedback which is likewise distinguished by a good noise behavior. This is because the capacitors used in this case also have very little noise. What is unfavorable, by contrast, is that the gain of the disclosed amplifier device having capacitive and negative feedback is greatly dependent on the frequency and on the load.
However, an amplifier device having a high bandwidth is favorable precisely when used in an ultrasonic apparatus, because of the requirements of being able to connect to different ultrasonic transducers, in particular those having center frequencies that differ from one another. The required bandwidth may in this case be of the order of magnitude of at least two decades.
Furthermore, the known amplifier devices often have a very high and occasionally also an undefined input impedance. The input impedance is typically at least 104 xcexa9. This has an unfavorable effect with regard to power matching of the amplifier device to a source resistance of the ultrasonic transducer connected to the input. This source resistance is of the order of magnitude of a few 10 xcexa9. If appropriate, the connected ultrasonic transducer may also be provided with a simple matching circuit which transforms the source resistance to a standard value of e.g. 200 xcexa9. However, this value is significantly below the value which is customary for the input impedance of the known amplifier devices. Apart from the power transfer which is not optimum in that case, the high or undefined input impedance can additionally also lead to a deterioration in the noise behavior. Furthermore, standing wave effects can occur, leading to undesirable distortion.
It is accordingly an object of the invention to provide an amplifier which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type in such a way that the gain of the amplifier is as independent of frequency as possible in a wide frequency range, and the amplifier has a defined real input impedance. Moreover, the intention is for the amplifier device to have as little noise as possible and to be readily integratable. In particular, the intention is also to eliminate the noise contribution of the negative feedback.
With the foregoing and other objects in view there is provided, in accordance with the invention, a low-noise, broadband amplifier, that includes a first broadband amplifier element having a first input, a second input, and a first output. A first negative feedback path is fed back from the first output to the first input, a second negative feedback path is fed back from the first output to the second input, and a controlled current source is disposed in the first negative feedback path.
In this case, the invention is based on the insight that, by means of a controlled current source in the first negative feedback path, it is possible both for the noise contribution of the second negative feedback path, required for setting the gain, to be reduced and for a real finite input impedance to be set. What has a particularly favorable effect in this case is that the setting of the real finite input impedance does not require an additional non-reactive resistor, the thermal noise of which would result in the noise behavior of the entire amplifier device also being impaired. Specifically, the controlled current source in the first negative feedback path has the effect that a virtual input impedance having a real and finite value forms at the input of the amplifier device. In particular, this does not require a resistive connection in parallel with the input of the amplifier device. The advantage of matching to the source resistance of a unit, for example an ultrasonic transducer, connected upstream of the amplifier device is thus accompanied by an improvement in the noise behavior.
Without any restriction to the general validity, the effect of the controlled current source in the first negative feedback path will now be explained for the case where the first amplifier element having negative feedback via the second negative feedback path is an operational amplifier connected up in a non-inverting fashion. The ideally infinite input impedance of the non-inverting operational amplifier can now be transferred by the controlled current source into an input impedance having a finite and real value. Specifically, the controlled current source converts the output voltage of the amplifier device into a current which appears, at the input of the amplifier device, as an input current which is proportional to the output voltage. On account of the proportional relationshipxe2x80x94determined by the gainxe2x80x94between the input voltage and the output voltage, a proportional relationship between the input current and the input voltage thus also results, that is to say a finite real input resistance.
Instead of the voltage-controlled current source used in the exemplary embodiment described above, however, it is equally possible to use a current-controlled current source. To that end, by way of example, a portion of the output voltage which is to be fed back in the manner of negative feedback is converted into a proportional current signal, which then serves as a control variable. Irrespective of the nature of its respective control, the current source can be embodied, without difficulty, with the high bandwidth required for the amplifier device.
The bandwidth of the amplifier device is otherwise determined principally by the first amplifier element, which, in particular, may be designed as a broadband transistor or operational amplifier. In the last-mentioned case, an embodiment is possible in the form of a component integrated on a chip or alternatively in the form of a discrete structure having a plurality of individual components. However, a bandwidth of at least two frequency decades can readily be realized in each embodiment.
In accordance with an added feature of the invention, a favorable embodiment variant is provided in which the current source is voltage-controlled. As already described above, this makes it possible, in a particularly simple manner, for an output voltage of a non-inverting operational amplifier to be converted into a finite input current of the amplifier device.
In accordance with an additional feature of the invention, a further embodiment variant is provided in which the first negative feedback path also includes a first voltage divider in addition to the controlled current source. Specifically, that portion of the current supplied by the controlled current source which appears at the input of the amplifier device can be set with the aid of the first voltage divider. The voltage divider also simultaneously serves as a current divider. The desired value for the real input impedance of the amplifier device can then be set very accurately by way of the divider ratio. It is particularly favorable if the first voltage divider includes at least a series circuit formed by a first and a second divider capacitor, since the divider capacitors do not generate thermal noise comparable to that of a non-reactive resistor. The frequency dependence of the two divider capacitors is not manifested on account of the behavior determined exclusively by the division ratio, so that practically no concessions at all result in the bandwidth which can be attained.
In accordance with another feature of the invention, a further embodiment is provided in which the second negative feedback path includes a second voltage divider, which sets the gain of the amplifier device independently of the optionally non-linear properties of the first amplifier element. The gain is determined exclusively by the ratio of the two impedances of the second voltage divider, the impedances being connected in series. One example of this is the operational amplifier connected up in a non-inverting fashion. A capacitively designed second voltage divider once again enables a particularly favorable noise behavior to be achieved. The second voltage divider then includes at least a series circuit formed by a third and a fourth divider capacitor. In order to prevent undesirable charging at the second input, the third and the fourth divider capacitors or just one of the two divider capacitors mentioned may, if appropriate, also be bridged resistively. In this case, this bridging may have an arbitrarily high resistance. The bridging resistors that may be present are unimportant for the noise since they are short-circuited by the third and/or fourth divider capacitor in the signal frequency range of interest.
In accordance with a further feature of the invention, another embodiment is provided in which the current source is controlled by means of a current. A current-inverting negative impedance converter, which is also referred to as INIC, is provided, in particular, for this purpose. The leading xe2x80x9cIxe2x80x9d in this case denotes current inversion and the rest is an abbreviation of the English term xe2x80x9cNegative Impedance Converterxe2x80x9d. An INIC is particularly well suited to use in the first negative feedback path, specifically because a current flows both from the input and from the output of the amplifier device into the first negative feedback path. Therefore, it is precisely an element having current inversion that is required. A circuit which, apart from an inductive transformerxe2x80x94which is undesirable on account of its large physical formxe2x80x94is constructed only with passive components cannot offer this function.
In accordance with another added feature of the invention, a further favorable variant is provided in which the current-inverting negative impedance converter includes a second broadband amplifier element, for example again in the form of an operational amplifier. The broadband embodiment ensures that the amplifier device overall can be used in a wide frequency range. A second output is fed back in each case to a third and a fourth input of the second amplifier element. The favorable current-inverting effect of the negative impedance converter is thereby achieved.
In accordance with another additional feature of the invention, another embodiment is provided in which the second negative feedback path includes a second voltage divider which, particularly when a current-inverting negative impedance converter is used in the first negative feedback path, may also be designed purely resistively with a series circuit formed by a first and second non-reactive divider resistor. The current-inverting negative impedance converter has the positive property of at least partly suppressing the noise contribution of the two non-reactive divider resistors. Moreover, the gain of the amplifier device can be set very accurately by means of the non-reactive divider resistors, which are also highly suitable for integration.
An embodiment in which the non-reactive divider resistors have an identical resistance is particularly advantageous. This embodiment is distinguished by the fact that the noise contributions of the two divider resistors and the voltage noise of the first amplifier element are completely suppressed by the current-inverting negative impedance converter.
In accordance with yet another added feature of the invention, there is provided a further embodiment in which a finite real input impedance is produced that can be set by appropriate connection of the two negative feedback paths. A finite real input impedance of 50 ohms or of 200 ohms is particularly favorable precisely in the case of an ultrasonic transducer connected upstream of the amplifier device because the source impedance of the ultrasonic transducer has this value or can be transformed without difficulty to these resistance values. Reflection-free matching is thus achieved in conjunction with low inherent noise of the amplifier device.
Because of the favorable noise behavior and the input impedance which can be adapted to a predetermined value of a source resistance, the amplifier device can be used particularly advantageously in an ultrasonic apparatus. It then serves, in particular, as a preamplifier which is matched to an ultrasonic transducer connected upstream and preamplifies an output signal of the ultrasonic transducer with particularly little noise for further processing. Use in a magnetic resonance apparatus or in a radio-frequency measuring apparatus is likewise possible. In these apparatuses, too, a good noise behavior and an adjustable real input impedance are favorable properties of a preamplifier that is used, particularly when these properties apply over a large frequency range, as in the case of the amplifier device.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a low-noise broadband amplifier device having negative feedback via a controlled current source, and use of the amplifier device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.